The present invention relates generally to data transmission in mobile communication systems and more specifically to a user equipment (UE) specific slot structure for Physical Uplink Control Channel (PUCCH) with transmit diversity to improve multiplexing capability.
As used herein, the terms “user equipment” and “UE” can refer to wireless devices such as mobile telephones, personal digital assistants (PDAs), handheld or laptop computers, and similar devices or other User Agents (“UAs”) that have telecommunications capabilities. A UE may refer to a mobile, or wireless device. The term “UE” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems/equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the phrase “base station” or “access device” will refer to any component, such as a traditional base station or an LTE or LTE-A base station (including eNBs), that can provide a UE with access to other components in a telecommunications system.
In mobile communication systems such as E-UTRAN, a base station provides radio access to one or more UEs. The base station comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UEs communicating with the base station. The functions of the scheduler include, among others, dividing the available air interface capacity between UEs, deciding the transport channel to be used for each UE's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared CHannel (PDSCH) and Physical Uplink Shared CHannel (PUSCH) data transmissions, and sends scheduling information to the UEs through a scheduling channel. In some cases, such as when no uplink application layer data is to be transmitted by the UE, control information is communicated from the UE to the base station using the PUCCH.
In LTE systems, data is transmitted to and from base stations and UEs on allocated Resource Blocks (RBs). In the LTE uplink, an RB, such as an RB used for PUCCH transmission, may be defined as a set of multiple consecutive subcarriers in the frequency domain and one or more consecutive symbols in the time domain. FIG. 1a is an illustration of several example PUCCH resource blocks (RBs). Generally, PUCCH RBs are located at or near the two extremes of available system bandwidth. Accordingly, referring to FIG. 1a, RBs 100 include two PUCCH RBs 102 that are allocated at the beginning of system bandwidth and two PUCCH RBs 102 that are allocated at the end of the available system bandwidth. The PUCCH for a single UE is transmitted over a single RB 102 at each of the two available slots 104 within the subframe. Note that here each PUCCH RB occupies all of one slot. Generally, for a UE, the assigned PUCCH RBs in the two slots of a subframe are allocated at opposite sides of the bandwidth. Accordingly, if a UE is assigned a first RB having index 1 (one greater than the lowest index of 0) in the first slot, the UE is also assigned the RB at index N−2 in the second slot. Accordingly, referring to FIG. 1a, a UE may be assigned RB 106 in addition to RB 108.
FIG. 1b is an illustration showing additional detail of the PUCCH RBs of FIG. 1a. RBs 100 each include several symbols formed in each of the two available slots. Each slot includes several symbols 110 that may each contain either data or a reference signal (RS). The RS may be used to measure channel conditions between the UE and a base station. In the present disclosure, the PUCCH RB structure is summarized as a plurality of individual slots.
In LTE-A, a UE may be configured to initiate simultaneous transmission from multiple antennas for uplink (UL) communications. When using multiple antennas, the communications may be referred to as multiple-input, multiple-output (MIMO) communications. By using MIMO, the signal strength and throughput between a UE and a base station can be improved. When using MIMO, a suitable transmit diversity (TxD) scheme can be used to ensure that communications transmitted by each antenna can be distinguished from one another. For example, the transmissions of each antenna may be encoded using different orthogonal sequences to make them more easily separable. Accordingly, the TxD scheme can be used to improve the coverage in an LTE-A system, to reduce required UE transmission power to reach a given level of coverage, and/or to reduce the interference caused by the transmissions.
In MIMO configurations, with respect to the PUCCH, although some TxD schemes provide improved performance when compared to single antenna transmissions, they suffer drawbacks in that each UE requires twice as many of the limited number of available orthogonal resources to enable the base station to receive and separate transmissions from each antenna accurately. As a result, the number of UEs that can be multiplexed for their PUCCH transmission within the same RB (that is, transmit on the same RB without causing excessive interference), is reduced by a factor of two when using two antennas as compared with transmissions using a single antenna.
Generally, in existing LTE uplink communications, the multiple-access method for the PUCCH is code division multiple access (CDMA). Using CDMA, several UEs transmit their PUCCHs using the same time-frequency resource blocks, but the transmissions are separated using UE-specific orthogonal sequence (OS) resources. The PUCCH can be configured in several different possible formats (some existing PUCCH configurations may have 6 different configurations). In one example PUCCH format designated format 2, the orthogonal resources are generated by applying cyclic shifting to a base sequence of length 12 with different cyclic shifts. As such, the orthogonal resources may be referred to as cyclic shift (CS) sequences or OS sequences. Accordingly, the number of these mutually orthogonal resources may be equal to 12.
For alternative PUCCH formats such as format 2, format 2a, and format 2b with normal cyclic prefixes (CP), of the seven symbols forming a transmission slot (see, for example, the RB of FIG. 1b), five symbols are used for data symbol (DS) transmission (see element 112 for example) and two symbols are used for reference symbol (RS) transmission (see element 114, for example). The RS may be used to measure and evaluate the quality of the radio link between the UE and the base station. In the case of extended CP, each slot contains five DSs and one RS.
FIGS. 2a and 2b are illustrations of example slot structures for format 2 PUCCHs. FIG. 2a is an illustration of the slot structure including a normal CP and FIG. 2b is an illustration of the slot structure including an extended CP. In each slot structure, the position of the DSs and RSs may be fixed as specified by a standard. The resource elements (e.g., subcarriers) at each DS or RS are filled using an appropriate CS sequence. For each DS, the corresponding CS sequence is multiplied by one of the symbols generated from the encoded data to be transmitted. The assignment of CS sequence to the symbols in each subframe may be configured by the base station and can be signaled to the UE using higher layer signaling. In any given RB of the PUCCH and at any time, each CS sequence can be used by at most one UE. Accordingly, in the existing configurations of LTE, the multiplexing capacity of PUCCH is limited to 12 UEs (using a single antenna) when 12 CS OSs are provided, meaning that a maximum of 12 UEs could multiplex and transmit their PUCCH on the same PUCCH RBs.
There are several schemes for providing TxD in PUCCH communications. Transparent schemes are those that use a single orthogonal sequence for the DSs of a PUCCH slot. In those schemes, the power resources of both transmit antennas are utilized while making the scheme transparent to the base station. Examples of this type of scheme include RF combining and slot-based precoding vector switching (PVS) as described in R1-090786, LG Electronics, “PUCCH TxD Schemes for LTE-A”, 3GPP TSG RAN WG1 #56, February 2009 and R1-091374, Nortel, “Evaluation of transmit diversity for PUCCH in LTE-A”, 3GPP TSG RAN WG1 #56b, March 2009. In these schemes, both transmit antennas use the same CS OS sequence. As a result, there is no need to signal a new sequence assignment to the UE and also the multiplexing capacity remains the same as in LTE Release 8 (Rel-8). However, while these schemes provide power pooling benefits, they provide little or no spatial diversity gain over single antenna transmission implementations.
In contrast, non-transparent schemes using a single OS for the DS, but different OSs for the RS may be implemented. Examples of this scheme include Space Time Block Code (STBC) based TxD schemes described in R1-090786, LG Electronics, “PUCCH TxD Schemes for LTE-A”, 3GPP TSG RAN WG1 #56, February 2009, R1-091374, Nortel, “Evaluation of transmit diversity for PUCCH in LTE-A”, 3GPP TSG RAN WG1 #56b, March 2009, and R1-094223, Qualcomm Europe, “Transmit Diversity for PUCCH Format 2/2a/2b”, 3GPP TSG RAN WG1 #58b, October 2009. In these schemes, both transmit antennas are configured to use the same orthogonal sequence for transmission of DSs. However, RS transmissions using different antennas use different OSs to allow for base station channel estimation to be performed for each antenna individually. In that case, because two OSs are needed for the two RSs transmitted by the two antennas, the multiplexing capacity of these schemes is reduced by a factor of two as compared with the single antenna transmission in Rel-8.
Alternatively, non-transparent schemes using two orthogonal sequences for both the DS and RS may be implemented. In these schemes, different transmit antennas use different orthogonal sequences for transmission of both DSs and RSs. Example of such schemes include Spatial Orthogonal-Resource Transmit Diversity (SORTD) in which the same modulated symbols are transmitted simultaneously from different antennas using different CS sequences. Example schemes are described in R1-090786, LG Electronics, “PUCCH TxD Schemes for LTE-A”, 3GPP TSG RAN WG1 #56, February 2009, R1-091374, Nortel, “Evaluation of transmit diversity for PUCCH in LTE-A”, 3GPP TSG RAN WG1 #56b, March 2009, R1-094223, Qualcomm Europe, “Transmit Diversity for PUCCH Format 2/2a/2b”, 3GPP TSG RAN WG1 #58b, October 2009, and R1-093052, Huawei, “Performance of UL multiple antenna transmission for PUCCH”, 3GPP TSG RAN WG1 #58, August 2009. The advantage of these schemes is that their performance is better than schemes that use the same OS on each antenna. However, as a result of using twice as many of the available orthogonal resources, the schemes' PUCCH multiplexing capacity is reduced by a factor of two as compared with the PUCCH multiplexing capacity of single antenna transmission in Rel-8.
Accordingly, in PUCCH transmissions using multiple antennas, there is a need for a TxD scheme that preserves the low peak-to-average power ratio (PAPR) property of the UL signal, makes efficient use of the power resources of both transmit antennas available to a UE, provides high PUCCH multiplexing capability, provides improved performance over single antenna transmissions and is backward compatible with existing network implementations (e.g., LTE Release 8).